Lactose
present, glucose scarce (cAMP level high):
lac transcription stimulated.
RNA polymerase has high affinity for the lac promoter
only when the activator, catabolite-activating protein (CAP),
is bound next to the promoter.
CAP
is active only when associated with cyclic AMP
(cAMP),
whose concentration in the cell rises when the concentration of the preferred glucose falls.